How should client-side and server-side form validation work together?

Use client-side validation for responsive feedback and server-side validation as the authoritative check. Share clearly defined field rules where practical, but still validate request types, lengths, allowed values, and cross-field business rules on the server. Retu structured field errors and preserve safe user input so users can correct mistakes. Database constraints provide another layer for invariants such as uniqueness. Consider race conditions: a use ame can become unavailable after a browser check. Validation errors should explain the correction without exposing inte al stack traces or sensitive records.